# Devel::Peek - A data debugging tool for the XS programmer # The documentation is after the __END__ package Devel::Peek; # Underscore to allow older Perls to access older version from CPAN $VERSION = '1.00_01'; require Exporter; use XSLoader (); @ISA = qw(Exporter); @EXPORT = qw(Dump mstat DeadCode DumpArray DumpWithOP DumpProg); @EXPORT_OK = qw(SvREFCNT SvREFCNT_inc SvREFCNT_dec CvGV); %EXPORT_TAGS = ('ALL' => [@EXPORT, @EXPORT_OK]); XSLoader::load 'Devel::Peek'; sub DumpWithOP ($;$) { local($Devel::Peek::dump_ops)=1; my $depth = @_ > 1 ? $_[1] : 4 ; Dump($_[0],$depth); } 1; __END__ =head1 NAME Devel::Peek - A data debugging tool for the XS programmer =head1 SYNOPSIS use Devel::Peek; Dump( $a ); Dump( $a, 5 ); DumpArray( 5, $a, $b, ... ); mstat "Point 5"; =head1 DESCRIPTION Devel::Peek contains functions which allows raw Perl datatypes to be manipulated from a Perl script. This is used by those who do XS programming to check that the data they are sending from C to Perl looks as they think it should look. The trick, then, is to know what the raw datatype is supposed to look like when it gets to Perl. This document offers some tips and hints to describe good and bad raw data. It is very possible that this document will fall far short of being useful to the casual reader. The reader is expected to understand the material in the first few sections of L. Devel::Peek supplies a C function which can dump a raw Perl datatype, and C function to report on memory usage (if perl is compiled with corresponding option). The function DeadCode() provides statistics on the data "frozen" into inactive C. Devel::Peek also supplies C, C, and C which can query, increment, and decrement reference counts on SVs. This document will take a passive, and safe, approach to data debugging and for that it will describe only the C function. For format of output of mstats() see L>. Function C allows dumping of multiple values (useful when you need to analize returns of functions). The global variable $Devel::Peek::pv_limit can be set to limit the number of character printed in various string values. Setting it to 0 means no limit. =head1 EXAMPLES The following examples don't attempt to show everything as that would be a monumental task, and, frankly, we don't want this manpage to be an internals document for Perl. The examples do demonstrate some basics of the raw Perl datatypes, and should suffice to get most determined people on their way. There are no guidewires or safety nets, nor blazed trails, so be prepared to travel alone from this point and on and, if at all possible, don't fall into the quicksand (it's bad for business). Oh, one final bit of advice: take L with you. When you return we expect to see it well-thumbed. =head2 A simple scalar string Let's begin by looking a simple scalar which is holding a string. use Devel::Peek; $a = "hello"; Dump $a; The output: SV = PVIV(0xbc288) REFCNT = 1 FLAGS = (POK,pPOK) IV = 0 PV = 0xb2048 "hello"\0 CUR = 5 LEN = 6 This says C<$a> is an SV, a scalar. The scalar is a PVIV, a string. Its reference count is 1. It has the C flag set, meaning its current PV field is valid. Because POK is set we look at the PV item to see what is in the scalar. The \0 at the end indicate that this PV is properly NUL-terminated. If the FLAGS had been IOK we would look at the IV item. CUR indicates the number of characters in the PV. LEN indicates the number of bytes requested for the PV (one more than CUR, in this case, because LEN includes an extra byte for the end-of-string marker). =head2 A simple scalar number If the scalar contains a number the raw SV will be leaner. use Devel::Peek; $a = 42; Dump $a; The output: SV = IV(0xbc818) REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42 This says C<$a> is an SV, a scalar. The scalar is an IV, a number. Its reference count is 1. It has the C flag set, meaning it is currently being evaluated as a number. Because IOK is set we look at the IV item to see what is in the scalar. =head2 A simple scalar with an extra reference If the scalar from the previous example had an extra reference: use Devel::Peek; $a = 42; $b = \$a; Dump $a; The output: SV = IV(0xbe860) REFCNT = 2 FLAGS = (IOK,pIOK) IV = 42 Notice that this example differs from the previous example only in its reference count. Compare this to the next example, where we dump C<$b> instead of C<$a>. =head2 A reference to a simple scalar This shows what a reference looks like when it references a simple scalar. use Devel::Peek; $a = 42; $b = \$a; Dump $b; The output: SV = RV(0xf041c) REFCNT = 1 FLAGS = (ROK) RV = 0xbab08 SV = IV(0xbe860) REFCNT = 2 FLAGS = (IOK,pIOK) IV = 42 Starting from the top, this says C<$b> is an SV. The scalar is an RV, a reference. It has the C flag set, meaning it is a reference. Because ROK is set we have an RV item rather than an IV or PV. Notice that Dump follows the reference and shows us what C<$b> was referencing. We see the same C<$a> that we found in the previous example. Note that the value of C coincides with the numbers we see when we stringify $b. The addresses inside RV() and IV() are addresses of C structure which holds the current state of an C. This address may change during lifetime of an SV. =head2 A reference to an array This shows what a reference to an array looks like. use Devel::Peek; $a = [42]; Dump $a; The output: SV = RV(0xf041c) REFCNT = 1 FLAGS = (ROK) RV = 0xb2850 SV = PVAV(0xbd448) REFCNT = 1 FLAGS = () IV = 0 NV = 0 ARRAY = 0xb2048 ALLOC = 0xb2048 FILL = 0 MAX = 0 ARYLEN = 0x0 FLAGS = (REAL) Elt No. 0 0xb5658 SV = IV(0xbe860) REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42 This says C<$a> is an SV and that it is an RV. That RV points to another SV which is a PVAV, an array. The array has one element, element zero, which is another SV. The field C above indicates the last element in the array, similar to C<$#$a>. If C<$a> pointed to an array of two elements then we would see the following. use Devel::Peek 'Dump'; $a = [42,24]; Dump $a; The output: SV = RV(0xf041c) REFCNT = 1 FLAGS = (ROK) RV = 0xb2850 SV = PVAV(0xbd448) REFCNT = 1 FLAGS = () IV = 0 NV = 0 ARRAY = 0xb2048 ALLOC = 0xb2048 FILL = 0 MAX = 0 ARYLEN = 0x0 FLAGS = (REAL) Elt No. 0 0xb5658 SV = IV(0xbe860) REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42 Elt No. 1 0xb5680 SV = IV(0xbe818) REFCNT = 1 FLAGS = (IOK,pIOK) IV = 24 Note that C will not report I the elements in the array, only several first (depending on how deep it already went into the report tree). =head2 A reference to a hash The following shows the raw form of a reference to a hash. use Devel::Peek; $a = {hello=>42}; Dump $a; The output: SV = RV(0xf041c) REFCNT = 1 FLAGS = (ROK) RV = 0xb2850 SV = PVHV(0xbd448) REFCNT = 1 FLAGS = () NV = 0 ARRAY = 0xbd748 KEYS = 1 FILL = 1 MAX = 7 RITER = -1 EITER = 0x0 Elt "hello" => 0xbaaf0 SV = IV(0xbe860) REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42 This shows C<$a> is a reference pointing to an SV. That SV is a PVHV, a hash. Fields RITER and EITER are used by C>. =head2 Dumping a large array or hash The C function, by default, dumps up to 4 elements from a toplevel array or hash. This number can be increased by supplying a second argument to the function. use Devel::Peek; $a = [10,11,12,13,14]; Dump $a; Notice that C prints only elements 10 through 13 in the above code. The following code will print all of the elements. use Devel::Peek 'Dump'; $a = [10,11,12,13,14]; Dump $a, 5; =head2 A reference to an SV which holds a C pointer This is what you really need to know as an XS programmer, of course. When an XSUB returns a pointer to a C structure that pointer is stored in an SV and a reference to that SV is placed on the XSUB stack. So the output from an XSUB which uses something like the T_PTROBJ map might look something like this: SV = RV(0xf381c) REFCNT = 1 FLAGS = (ROK) RV = 0xb8ad8 SV = PVMG(0xbb3c8) REFCNT = 1 FLAGS = (OBJECT,IOK,pIOK) IV = 729160 NV = 0 PV = 0 STASH = 0xc1d10 "CookBookB::Opaque" This shows that we have an SV which is an RV. That RV points at another SV. In this case that second SV is a PVMG, a blessed scalar. Because it is blessed it has the C flag set. Note that an SV which holds a C pointer also has the C flag set. The C is set to the package name which this SV was blessed into. The output from an XSUB which uses something like the T_PTRREF map, which doesn't bless the object, might look something like this: SV = RV(0xf381c) REFCNT = 1 FLAGS = (ROK) RV = 0xb8ad8 SV = PVMG(0xbb3c8) REFCNT = 1 FLAGS = (IOK,pIOK) IV = 729160 NV = 0 PV = 0 =head2 A reference to a subroutine Looks like this: SV = RV(0x798ec) REFCNT = 1 FLAGS = (TEMP,ROK) RV = 0x1d453c SV = PVCV(0x1c768c) REFCNT = 2 FLAGS = () IV = 0 NV = 0 COMP_STASH = 0x31068 "main" START = 0xb20e0 ROOT = 0xbece0 XSUB = 0x0 XSUBANY = 0 GVGV::GV = 0x1d44e8 "MY" :: "top_targets" FILE = "(eval 5)" DEPTH = 0 PADLIST = 0x1c9338 This shows that =over =item * the subroutine is not an XSUB (since C and C are non-zero, and C is zero); =item * that it was compiled in the package C
; =item * under the name C; =item * inside a 5th eval in the program; =item * it is not currently executed (see C); =item * it has no prototype (C field is missing). =back =head1 EXPORTS C, C, C, C, C and C by default. Additionally available C, C and C. =head1 BUGS Readers have been known to skip important parts of L, causing much frustration for all. =head1 AUTHOR Ilya Zakharevich ilya@math.ohio-state.edu Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. Author of this software makes no claim whatsoever about suitability, reliability, edability, editability or usability of this product, and should not be kept liable for any damage resulting from the use of it. If you can use it, you are in luck, if not, I should not be kept responsible. Keep a handy copy of your backup tape at hand. =head1 SEE ALSO L, and L, again. =cut